refactor: standardize dark theme selectors and add CSS variables

CSS improvements:
- Standardize dark theme selectors to body[class*="dark"] pattern
- This ensures all dark themes (theme-dark, theme-dracula, etc.)
  receive consistent styling
- Add semantic color variables (--text-primary, --bg-primary, etc.)
- Replace hardcoded colors with CSS variables in:
  - Tab bar component
  - Toolbar separator
  - Pane resizer
  - Status bar
- Add fallback values for backward compatibility

This improves maintainability and makes theming more consistent.

Amit Haridas

src/styles-modern.css

@@ -16,6 +16,12 @@
   --accent-purple: #8b5cf6;
   --accent-pink: #ec4899;
 
+  /* Semantic Colors */
+  --success: #10b981;
+  --warning: #f59e0b;
+  --error: #ef4444;
+  --info: #3b82f6;
+
   /* Neutral Colors */
   --gray-50: #f9fafb;
   --gray-100: #f3f4f6;
@@ -28,6 +34,19 @@
   --gray-800: #1f2937;
   --gray-900: #111827;
 
+  /* Semantic UI Colors (Light Theme Default) */
+  --text-primary: #1f2937;
+  --text-secondary: #6b7280;
+  --text-muted: #9ca3af;
+  --bg-primary: #ffffff;
+  --bg-secondary: #f9fafb;
+  --bg-tertiary: #f3f4f6;
+  --border-color: #e5e7eb;
+  --border-color-strong: #d1d5db;
+  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
+  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1);
+  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.1);
+
   /* Glassmorphism */
   --glass-bg: rgba(255, 255, 255, 0.7);
   --glass-border: rgba(255, 255, 255, 0.18);